Firstly, it's a legal, or at least a moral, issue -- you promised to pay them, so you should. Secondly, get a rep for not paying your soundmen and you might find them hard to come by. Thirdly (unless you're very small-scale), the soundman is a really important part of the band and should be looked after (but they're not actually in the band, which is why you have to pay them).
I've even paid soundguys for helping out with "dress rehearsals" where we have gone to a local club or hall and set up all the gear to rehearse in a more realistic environment and to check that everything is working properly.
I also pay soundguys who do nothing but turn up just before the gig, twiddle the knobs and go away, although they don't get as much as the guys who help lug gear in and set up and break down afterwards.
So if I was you I would blow these guys off right now, and tell them to stick their 5 FX units up their behinds. But it's clear that you want to give them another chance. Make sure that's their last chance.